ELEKIT
TU-8100 ($299 and their NLA wood speaker (below) made a showing here at CAS
2014.

ELEKIT's TU-HP01 is a portable headphone amplifier with a hybrid
amplification configuration. There are a pair of Raytheon 6418 subminiature tubes for input and OP-amp for output. You can power it with the excellent
Eneloop AA batteries (or any other AA, yet Eneloop are my personal faves). The Raytheon 6418 is very low in power consumption and microphonics too. There are a pair
of JRC MUSES8820 and a BurrBrown OPA2604 within this portable design. Frequency response is from 10 Hz to 65 kHz and headphone impedance is from 16 to 32 Ohm (recommended) via 3.5mm stereo mini headphone jack.